The objective of the proposed experiments is to improve the usefulness of gallium-67 as a tumor scanning agent by answering the following question: What aspect of tumor biology confers specificity on a tumor, such that it concentrates gallium-67 to a greater extent than surrounding normal tissue or another tumor? We will make use of a unique tumor model, the EMT-6 sarcoma of BALB/c mice. This tumor grows both in vivo as a transplantable subcutaneous tumor and in vitro as a tissue monalayer. Using this model we can examine tumor uptake of radiogallium more completely than has been previously possible. Our preliminary work with this system suggests that a tumor-associated "transferrin receptor" is the functional unit responsible for the affinity of gallium for certain neoplasms. This receptor also appears to be active in tumor uptake of iron. Accordingly, our proposed studies of cellular uptake and subcellular incorporation of radiogallium will focus on a) the role of transferrin and a transferrin-specific cellular receptor, and b) the interrelationship to iron metabolism. Our study plan involves: (1) In vitro uptake studies in tissue culture, which will better characterize radiogallium uptake in comparison to uptake of iron-59 (59Fe) and iodine-125 labeled transferrin (125-TF); (2) Subcellular fractionation of distilled water tumor extracts, in order to determine the subcellular fate of 67Ga, in comparison to 59Fe and 125I-TF subsequent to uptake, both for in vitro and in vivo tumor growth forms; (3) Stable gallium cytotoxicity for EMT-6 as modified by iron and transferrin. These studies will provide an additional end point (reproductive integrity) for evaluation of the interaction between gallium and iron; (4) Transferrin receptor characterization, using techniques analogous to those described for studying transferrin receptors on bone marrow cells, to isolate a partially purified transferrin receptor component from EMT-6 mouse tumor; (5) Comparison studies of radiogallium and radioiron uptake in normal tissues and other tumors, including mouse liver, Chang liver cells (in culture), reticulocyte rich blood cells, and a malignant human histiocytoma.